The distribution of virions, sites of virus multiplication (viroplasmas), and accumulation of yellow fever virus will be studied in various genetic strains and isolates of Aedes aegypti by means of electron microscopy. The mosquitoes will be inoculated mechanically and by feeding with a virulent strain, then dissected at intervals and studied by EM techniques. Immunofluorescence studies will be used parallel with EM observations. The ability of mosquitoes of diverse origin to transmit yellow fever virus will be tested. The study is expected to provide data concerning the differential susceptibility of A. aegypti strains, races, or geographic isolates of the yellow fever virus in an attempt to explain the lack of yellow fever outbreaks in areas of the world where the vector is prevalent.